In total knee joint replacement surgery, a surgeon typically affixes two prosthetic components to a patient's bone structure: a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component, respectively.
The femoral component is placed on a patient's distal femur after appropriate resection. One common type of femoral component, the condylar component, features a J-shaped cross section, with an anterior face and two condylar portions. The femoral component is usually attached to a femoral stem which is received in the patient's intramedullary femoral canal.
A common type of tibial component uses a tray or plate that generally conforms to the patient's resected proximal tibia. The tibial component is usually attached to a tibial stem which is received in the patient's intramedullary tibial canal.
The tibial plateau and the condyles of the femur bearing on the tibial plateau act similar to a hinge within the knee to allow bending and movement of the knee. The tibial component and the femoral component ultimately cooperate with each other to replicate as closely as possible the action and relationship of the tibial plateau and the condyles of the femur bearing on it. A plastic or polymeric (often ultra high molecular weight polyethylene or UHMW) insert or bearing may fit between the plate of the tibial component and the femoral component. This insert or bearing provides a surface against which the femoral component condylar portions articulate (i.e., move in gross motion corresponding generally to the motion of the femur relative to the tibia).
Accurately positioning and fitting the prosthetic components is important for a number of reasons. Each patient has a different bone structure and geometry. Dynamically, motion of the tibia relative to the femur about every axis varies from one patient to the next. Even though the surgeon uses various imaging techniques and palpation to study a particular patient's anatomy, she nevertheless gains considerable additional information required to fit the prosthetic components after the knee has been surgically exposed and surgery is underway.
Trial prostheses are conventional for, among other things, trying the fit of prosthesis or implant components to respective portions of the joint. After shaping the femur and the tibia, the surgeon may temporarily fit trial components instead of the actual prosthetic components to the femur and/or tibia, respectively. This enables the surgeon to test the fit of the components to the femur and tibia and to test their performance both statically and dynamically throughout a desired range of motion. Use of trial prosthetics instead of actual implants also allows the surgeon to perform this testing and achieve a more perfect fit and a more accurate performance of the actual implant component without introducing a number of “new” actual prosthetic components into the surgical field.
Using actual, final prosthetic components for this fitting procedure is undesirable. Using trial prosthetic components instead of the actual implants allows the surgeon to position, move, and fit components while trying various sizes and, if desired, while modifying bone structure, without imparting wear and tear on actual implant components—upon which destruction could have adverse long-term effects. Additionally, the use of trial components keeps the implants from requiring re-sterilization if they are used and a new size is needed. Therefore, trial components, such as trial tibial components, trial femoral components, and trial stems, are initially used. The actual tibial and femoral implants are then assembled based on these trial components and implanted into the knee.
When a stem that is not offset is used and an offset is needed, the outside component can overhang or underhang, and thus adjustability is needed based on individual anatomy.
In addition to being offset from the mechanical axis, the tibial and femoral canals may be angled with respect to the mechanical axis of the leg. Across a population of humans, a valgus bowing of the tibia exists relative to the mechanical axis. Consequently, if a stem oriented parallel to the mechanical axis of the leg is inserted into a bowed tibial canal, the stem can impinge on the lateral cortex of the tibial canal proximal to the knee and the medial cortex distal to the knee. Similarly, the femoral canal can bow posteriorly relative to the mechanical axis, which results in impingement by the stem of the anterior cortex of the femoral canal in the diaphysis of the femur and the posterior cortex slightly superior to the knee. Such impingement can prevent adequate penetration of the canal by the stem and result in improper positioning of the tibial and femoral components in the knee.
Improper positioning of the component with respect to the bone can have adverse effects, including stress shielding and bone loss due to non-uniform transfer of load from the bone to the stem. It can also limit range of motion. Insertion of a stem into an angled tibial canal may result in misalignment of the tibial component with the tibial plateau so that a part of the tibial component hangs over the tibial plateau. Such overhang can lead to the tibial component rubbing the soft tissue surrounding the knee, causing irritation and pain. Moreover, a consequence of overhang by one side of the tibial component is underhang by the other side of the tibial component, so that the underhang portion of the component is resting on the softer cancellous bone instead of the harder cortical bone along the peripheral rim of the tibial plateau. The component may consequently sink into the softer bone, causing the entire component to tilt toward the side of the underhang. This can jeopardize the stability of the implant.
To accommodate such offset and/or bowed canals, stem extensions have been designed to connect tibial and/or femoral components to corresponding stems using rotational adjustment systems. These rotational adjustment systems result in a trial and error process where the surgeon sets the stem extension to a predetermined, discrete position and inserts the components into the patient's canal to determine if the assembly fits correctly. If the trial prosthesis with the stem extension does not align with the geometry of the patient's intramedullary canal, the surgeon removes the components from the patient's canal, resets the rotation position, and repositions the stem extension at a different preset position. This process is repeated until one of the preset positions best aligns with the geometry of the patient's canal. Such a trial and error process increases the time spent in the operating room and increases the possibility of damage to the bone due to repeated entry and exit of the intramedullary canal, as well as increased opportunity for infection. Moreover, other systems that do not use preset positioning also have problems because they fail to provide a way to lock the desired orientation. Accordingly, it is likely that the orientation determined using such systems will be disrupted when the trial prosthesis is removed from the patient. If this occurs, the final implant that is constructed based on the trial prosthesis will not be oriented correctly. Thus, such systems do not always allow for a perfect match with the anatomy of the patient's intramedullary canal, and improvements are necessary.